Mechanistic Insights into Atmospheric Acid-Catalyzed Liquefaction of Corn Stalk: A Multidimensional Analysis of Bio-Oil Formation Pathways and Solid Residue Evolution

Significant progress has been made in the preparation of fossil fuel substitutes and high-value chemicals by the atmospheric-pressure catalytic liquefaction of lignocellulosic biomass. However, the time-dependent mechanisms governing the evolution of liquid- and solid-phase products during this process remain poorly understood, particularly the synergistic interactions between phase-specific transformation pathways at different reaction durations. This study systematically investigates the atmospheric acid-catalyzed liquefaction process of corn stalk (CS) employing a H3PO4-catalyzed binary solvent system (1,2-propanediol/diethylene glycol, PG/DEG), with particular focus on elucidating the time-dependent phase evolution mechanisms to advance the production of liquid fuels and high-value chemicals. Speculation was made on the liquefaction pathway of CS (three components) by analyzing the products at different liquefaction times. Time-dependent analysis revealed a staged transformation mechanism: Initial hydrogen (H•, HO•) donation from PG/DEG facilitated radical-driven depolymerization, generating low-molecular-weight (LMW) intermediates that are critical for bio-oil formation. Under optimized conditions (170 °C, 45 min residence time), achieving the highest liquefaction yield of 99.5%, subsequent temperature-sensitive processes (dehydration, isomerization, and bond cleavage) promoted conjugated structure formation (C = O, C = C). Radical-mediated bond scission (C–O–C, C–O, and C–C) generated oxygenated compounds, with 68 distinct species identified by GC-MS, including phenols, ketones, acids, and esters. Acid formation pathways were uniquely traced to ketone decarbonylation/oxidation, followed by alcohol esterification. Simultaneous aromatic derivative formation occurred through the selective phenylpropane framework degradation. Late-stage cross-polymerization between reactive intermediates and solvents formed insoluble residues via radical recombination.